utils/tar: fix validation for tar without directory or extensions

[cho-m]

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While running brew bump-cask-pr ... I have seen a tar validation issue a few times.
For example:

==> Verifying checksum for 'f4a5f395b90a4e7a31973a859427284a3c6bec550dbc480a3c588bb68d3dd978--helix-core-server.tgz'
Warning: Cannot verify integrity of 'f4a5f395b90a4e7a31973a859427284a3c6bec550dbc480a3c588bb68d3dd978--helix-core-server.tgz'.
No checksum was provided for this resource.
For your reference, the checksum is:
  sha256 "9a1f528f92568531785b1f125e60a48e84b1a958f9e1d74dfc6e61c060e08721"
Error: /Users/cho-m/Library/Caches/Homebrew/downloads/f4a5f395b90a4e7a31973a859427284a3c6bec550dbc480a3c588bb68d3dd978--helix-core-server.tgz is not a valid tar file!

The problem seems to be that the validation of tar file depends on regex /.*\., which means that a directory must exist and at least one file must have an extension.

This shouldn't be a restriction on validity of tar file.

I decided to use logic similar to unpack:

brew/Library/Homebrew/unpack_strategy/tar.rb

Lines 32 to 34 in 5115cc2

	# Check if `tar` can list the contents, then it can also extract it.
	stdout, _, status = system_command("tar", args: ["--list", "--file", path], print_stderr: false)
	status.success? && !stdout.empty?

[BrewTestBot]

Review period will end on 2021-06-01 at 19:54:04 UTC.

[MikeMcQuaid]

One suggested tweak but: nice work here!

[MikeMcQuaid]

Suggested change

	odie "#{path} is not a valid tar file!" if !status.success? || stdout.empty?
	odie "#{path} is not a valid tar file!" if !status.success? || stdout.blank?

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks again @cho-m!